Absorption in the small intestine

Absorption in the small intestine

1. Villi

   • Thin walls, lined with epithelial cells
     • Short diffusion distance
   • Finger-like projections of the small intestine
     • Increases surface area for absorption by diffusion
   • Has a rich network of capillaries
     • Carry's away absorbed molecules to maintain a concentration gradient
   • Able to move
     • Maintains steep concentration gradient
   • Epithelial lining has microvilli
     • Increases surface area

2. Role of diffusion in absorption

   • Net movement of molecules/ions from a region where they are highly concentrated to a region when their concentration is lower
   • Concentration of glucose is greater in the lumen of the small intestine than in the blood
     • Concentration gradient
       • Maintained by circulation of blood
       • Villi muscles contract and relax, mixing contents of small intestine
         • Glucose-rich food is next to villi

3. Role of active transport in absorption

   • Diffusion only results in equal concentrations of glucose on either side of the intestinal epithelium
     • Not all available glucose is absorbed
   • Glucose is absorbed by co-transport, it is drawn in with sodium ions (Na+)
   • Mechanism
     • 1. The sodium-potassium pump transported 2 potassium ions in and 3 sodium ions out (to blood)
     • 2. There is now a much higher concentration of Na+ in the lumen than epithelial cells
     • 3. Sodium ions diffuse into the cell down their concentration gradient through a co-transport protein
     • 4. As they move in they couple with glucose molecules, drawing them into the epithelial cell
     • 5. Glucose then passes into the blood by facilitated diffusion
   • As sodium moves down it's conc. gradient, glucose moves up it's
     • ATP is used in Na+/K+ pump
       • Indirect active transport of glucose

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